This invention relates to an actuator for a rotary plug valve (or rotary ball valve) which utilizes a ball screw mechanism. An associated rotary plug valve having the rotary plug valve actuator and an associated method are also disclosed.
Rotary plug valves and rotary ball valves are well known and have wide application in the control of fluids in various industrial settings. The construction of the rotary plug valve usually takes the form of a cast valve body that defines an enclosed valve seat having both inlet and outlet ports. In a conventional ball valve, a spherical ball is rotatably placed in the valve seat, the spherical ball having a passageway through its diameter to provide for the flow of fluid from the inlet to the outlet side of the valve. A plug valve is constructed similarly, only the plug is not spherical. See U.S. Pat. No. 4,802,652, the disclosure of which is incorporated by reference herein.
The ball or plug is rotated by means of an operator mechanism, sometimes called an actuator. One type of actuator is shown in U.S. Pat. No. 4,802,652 which simply provides a handle for applying torque to the ball or plug. For larger plug or ball valves, a mechanism is typically used to apply the torque. One known mechanism is a worm gear actuator. As is well known, the worm gear mechanism consists of a screw shaft ("worm") which is engaged by a rotating gear.
The worm gear mechanism, while effective in increasing the mechanical advantage of the actuator, has numerous disadvantages. A first disadvantage is that the worm gear mechanism involves sliding motion between the worm and the gear. This means that much of the torque applied to the gear is lost due to friction, thus making the worm gear mechanism, even when new, difficult to turn. Of course, as the worm gear mechanism ages and rusts, it is even more difficult to turn. A second disadvantage is that the worm and gear are often exposed to dust, dirt and moisture, making it difficult to keep lubricant thereon with the attendant greater difficulty in turning the worm gear mechanism.
A ball screw mechanism is a known mechanical device that consists of a rotatable shaft having disposed thereon a ball screw nut. See, e.g., U.S. Pat. No. 3,628,397; Warner Electric Catalog (1983); and Noor Industries Catalog (1991), the disclosures of all three of which are incorporated herein by reference. The ball screw mechanism uses rolling contact (similar to ball bearings) instead of the sliding contact utilized in the worm gear mechanism. Thus, the ball screw mechanism is much more efficient than the worm gear mechanism.
Despite the long existence of the ball screw mechanism, no one, until now, has suggested its use in association with ball or plug valves.
Thus, there remains a need for a rotary valve actuator that is efficient and avoids the problems of the prior art worm gear mechanism.